The amount of data communicated has increased with the spread of the Internet. In order to cope with this trend, it is necessary to increase the capacity of a transmission channel. One of techniques for realizing an increase in capacity includes a quadrature amplitude modulation (QAM) system. Optical signals on which modulation of a QAM system is performed in a transmitter are demodulated in an optical receiver of a digital coherent system.
In optical communication of a QAM system, there is a large problem of a non-linear effect which is received when the optical signal propagates through a transmission channel. When the optical signal receives a non-linear effect in the transmission channel, the phase of the optical signal is rotated. Since the QAM system deals with complicated phase information, the reception of phase rotation due to a non-linear effect causes correct phase information not to be capable of being demodulated at the time of reception of the signal.
A method of solving such a problem includes a non-linear compensation system called back propagation. Such a compensation system is a system for compensating for waveform distortion while retracing propagation waveform from a receiving side to a transmitting side by performing dispersion compensation little by little and performing non-linear compensation immediately after each dispersion compensation.
However, in the back propagation, when one non-linear compensation stage is configured by combining a dispersion compensation function and a non-linear compensation function, it is necessary to increase the number of non-linear compensation stages in order to increase compensation capability. The dispersion compensation function is realized by a linear distortion compensation circuit, and the non-linear compensation function is realized by a non-linear distortion compensation circuit. The linear distortion compensation circuit includes an FFT/IFFT circuit in order to perform dispersion compensation in a frequency domain. The FFT/IFFT circuit has a large circuit size, and thus considering the mounting area or power consumption of an LSI, only several FFT/IFFT circuits can be mounted in one signal processing device.
On the other hand, Non-Patent Document 1 discloses a compensation system called filtered back propagation. Filtered back propagation is for reducing the number of non-linear compensation stages by using a time-averaged amount of the amount of phase rotation, calculated from signal strength, in non-linear compensation.
In addition, Non-Patent Document 2 discloses perturbation back propagation as a system for further reducing the number of non-linear compensation stages. In perturbation back propagation, even phase rotation caused by a perturbative component is canceled out in addition to the canceling out of phase rotation caused by the above-mentioned signal strength, thereby allowing the number of non-linear compensation stages to be reduced to, for example, three.